Communication systems based on radio signals have existed for over a century. Radio signals are electromagnetic waves, and designers of antennas for such signals have generated a wide range of designs with the goal of achieving good performance in a variety of operating conditions. Similarly, communication systems based on magnetic coupling have existed for a long time. Signals used by such systems (hereinafter “magnetic signals”) are magnetic fields. Antennas for magnetic signals are different from antennas for radio signals; however, the goal of achieving good performance in a variety of operating conditions is still important.
In both types of communication systems, the antenna couples energy into or out of the associated signal. Generally, an important goal for the antenna designer when designing, for example, a receiving antenna, is to maximize energy transfer between the signal incident on the antenna and the resulting electrical signal generated by the antenna. The higher the energy transfer, the higher the received signal-to-noise ratio will be, and this usually results in better receiver performance.
In many applications, small size is desirable. For example, RFID devices are usually very small. Some RFID devices use radio communications while others use magnetically-coupled communications. The two types of communications offer different advantages and disadvantages and, depending on the application, one or the other type of communications might be better.
In some applications, it is desirable to have a communication system that can implement both radio communications and magnetically-coupled communications, such that either type of communications can be used as needed. Such a communication system needs to have an antenna for radio signals (hereinafter “radio antenna”) and an antenna for magnetic signals (hereinafter “magnetic antenna”). If small size is also desirable, the requirement of having both an efficient radio antenna and an efficient magnetic antenna makes it difficult to simultaneously achieve small size and good communication performance with both types of communications.
It is important to underscore the fundamental difference between radio communications and magnetically-coupled communications. A radio signal is an electromagnetic wave that propagates through air or vacuum. As such, it comprises both magnetic fields and electric fields and the interrelationship between the electric and magnetic fields is what enables an electromagnetic wave to propagate through air or vacuum. In particular, the time-varying electric fields in the electromagnetic wave generate time-varying magnetic fields, and the time-varying magnetic fields generate time-varying electric fields, and the sequence then repeats. This chain of intertwined electric and magnetic fields is what enables an electromagnetic wave to carry energy that can be collected by a receiving radio antenna at large distances.
In contrast to radio signals, magnetic signals in magnetically-coupled communications consist exclusively of magnetic fields. Electric fields do not play a role. Typically, there is a first magnetic antenna implemented as a magnetic coil that generates a time-varying magnetic field, and there is a second magnetic antenna also implemented as a magnetic coil that picks up part of that magnetic field. Magnetic fields carry energy and, therefore, energy is transferred from the first coil to the second coil; however, the ability to transfer energy goes only as far as the magnetic field can reach. Even though the time-varying nature of the magnetic fields used for magnetic communications means that some incidental time-varying electric fields are generated, such electric fields are weak and hold only a very small amount of energy that is not useful for communication.
Because magnetic fields become weaker with distance faster than electromagnetic waves, radio communications are generally more advantageous at comparatively longer distances, and magnetically-coupled communications are more advantageous at comparatively shorter distances. Operation of a wireless system at the longer distances is often referred to as “long range” operation, while operation at the shorter distances is often referred to as “proximity” operation.
Generally, magnetically-coupled communications utilize frequencies that are significantly lower than the frequencies utilized by radio communications. Cost and ease of implementation depend on frequency as well as on the type of communication used. In certain applications, it is desirable to communicate through multiple frequencies, with some frequencies used for radio communications and other frequencies used for magnetically-coupled communications. Systems for such applications require antennas that are efficient at multiple frequencies.
In this specification, unless otherwise indicated, the verb “To conduct” and its inflected and derived forms (“conductor”, “conductive”, “conductance”, “conductivity”, etc.) refers to electrical conduction.
FIG. 1 depicts monopole radio antenna 100 in accordance with the prior art. Monopole radio antenna 100 comprises monopole 110, ground plane 115, and transmission line 130 which connects monopole 110 to input-output port 140. Monopole radio antenna 100 is an example of a radio antenna that comprises a flat metal sheet; in particular, ground plane 115 is a flat metal sheet and monopole 110 is a metal rod that protrudes out of a small hole in the metal sheet. Monopole 110 is supported in the small hole by bushing 150, which is an electrical insulator. Monopole 110 is perpendicular to ground plane 115.
Efficient operation of monopole radio antenna 100 depends on the fact that ground plane 115 is a good conductor at the frequency of operation of the antenna. Ground plane 115 is made out of a metal, such as aluminum, brass or copper, that is known to be a good conductor at the frequency of operation of the antenna. Operational parameters of the antenna, such as impedance, radiation pattern, et cetera, are a function of the shape, size, and electrical properties of ground plane 115. It is well known in the art how to predict such antenna parameters based on the specific shape, size and electrical properties of ground plane 115. It is also well known in the art how to adjust the shape, size and electrical properties of ground plane 115 so as to achieve desired results for such parameters.
FIG. 2 depicts RFID device 200 in accordance with the prior art. RFID device 200 comprises antenna 210 and load element 220. Antenna 210 is a radio antenna made out of a metal sheet bent into the shape depicted in FIG. 2. That shape forms resonant structure 250 which is known in the art to provide good coupling with electromagnetic radio signals near a specific frequency of resonance. Load element 220 is connected to the metal sheet at connection points 230-1 and 230-2 which, together, make up input-output port 240, which is the single input-output port of antenna 210.
Load element 220 is an electrical circuit for processing electrical signals generated by the antenna, and for generating electrical signals to be fed to the antenna. In a typical application, RFID device 200 receives a first radio signal whose energy is converted into a first electrical signal generated by the antenna. The first electrical signal appears at input-output port 240 and is processed by load element 220, which, in response, generates a second electrical signal. Load element 220 feeds this second electrical signal into input-output port 240, and antenna 210 converts the energy of the second electrical signal into a transmitted radio signal.
In some applications, load element 220 might be a single integrated circuit; in other applications, it might be a more complex device comprising a plurality of electronic components. In all applications, the impedance that load element 220 presents at its two terminals is crucial for proper operation of RFID device 200. Generally, when load element 220 is operated as a receiver for processing the first electrical signal, it is desirable for it to present an impedance that approximates a good conjugate match for the impedance of resonant structure 250 at the frequency of operation of RFID device 200. Such an impedance achieves maximum power transfer from the antenna to load element 220. However, in other applications, a different impedance might be more advantageous. For example, in applications where the first electrical signal needs to be rectified by a diode, it is important to achieve a voltage high enough to drive the diode into forward conduction. In such applications, a larger impedance that achieves higher voltages might be advantageous even though it might not achieve maximum power transfer.
When load element 220 is operated as a transmitter for generating the second electrical signal, yet other impedance values might be desirable. For example, for so-called backscatter modulation, load element 220 might vary its impedance between two values that are both substantially different from a conjugate match at the frequency of operation of RFID device 200. The impedance that load element 220 presents at frequencies other than the frequency of operation of RFID device 200 is, of course, inconsequential.
Much like the monopole antenna of FIG. 1, efficient operation of antenna 210 depends on the fact that the metal sheet is a good conductor at the frequency of operation of the antenna. Also, here too, operational parameters of the antenna, such as impedance, radiation pattern, et cetera, are a function of the shape, size, and electrical properties of the metal sheet out of which antenna 210 is made. It is well known in the art how to predict such operational parameters of the antenna based on the specific shape, size and electrical properties of the metal sheet. It is also well known in the art how to adjust the shape, size and electrical properties of the metal sheet so as to achieve desired results for such parameters.
FIG. 3 depicts RFID device 300 in accordance with the prior art. RFID device 300 comprises a dipole antenna that comprises two arms, which are depicted in FIG. 3 as 310-1 and 310-2. Each of the two arms is a metal sheet. RFID device 300 also comprises support substrate 360 which supports the two arms of the dipole antenna. Support substrate 360 is made out of a dielectric material and provides mechanical support for the entire structure. The use of support substrate 360 makes it possible for arms 310-1 and 310-2 to be thin metal layers deposited on the substrate. The substrate might be part of a printed-circuit board, and the thin metal layers might be copper traces on the printed-circuit board. The entire RFID device 300 might be part of a larger circuit on a printed-circuit board.
RFID device 300 also comprises load element 320, which is connected to the dipole antenna through input-output port 340. Similar to FIG. 2, load element 320 is an electrical circuit for processing electrical signals generated by the antenna, and for generating electrical signals to be fed to the antenna. The comments that were made in conjunction with FIG. 2 regarding load element 220 also apply for load element 320.
Much like the antennas of FIGS. 1 and 2, efficient operation of the dipole antenna in FIG. 3 depends on the fact that the metal sheets are good conductors at the frequency of operation of the antenna. Operational parameters of the antenna, such as impedance, radiation pattern, et cetera, are a function of the shape, size, electrical, and dielectric properties of the metal sheets and of the substrate. It is well known in the art how to predict such operational parameters of the antenna based on the specific shape, size, electrical, and dielectric properties of the metal sheets and of the substrate. It is also well known in the art how to adjust the shape, size and electrical properties of the metal sheets and of the substrate so as to achieve desired results for such parameters.
FIG. 4 depicts RFID device 400 in accordance with the prior art. RFID device 400 comprises a patch antenna that comprises two metal sheets; In particular, the antenna comprises floating patch 410 and ground plane 415, which are both metal sheets. RFID device 400 also comprises load element 420, which is connected to the dipole antenna through input-output port 440. Similar to FIG. 2, load element 420 is an electrical circuit for processing electrical signals generated by the antenna, and for generating electrical signals to be fed to the antenna. The comments that were made in conjunction with FIG. 2 regarding load element 220 also apply for load element 420.
In FIG. 4, floating patch 410 is depicted as floating in space without any mechanical support. In practice, the space between floating patch 410 and ground plane 415 might be filled with a dielectric material which provides mechanical support and whose dielectric properties affect the operational parameters of the antenna.
Much like the antennas of FIGS. 1, 2, and 3, efficient operation of the patch antenna in FIG. 4 depends on the fact that the metal sheets are good conductors at the frequency of operation of the antenna. Operational parameters of the antenna, such as impedance, radiation pattern, et cetera, are a function of the shape, size, and electrical properties of the metal sheets, as well as of the properties of any dielectric material between the two sheets, if present. It is well known in the art how to predict such operational parameters of the antenna based on the specific shape, size, and electrical properties of the metal sheets, and of any dielectric material between the two sheets, if present. It is also well known in the art how to adjust the shape, size and electrical properties of the metal sheets, and of any dielectric material between the two sheets, if present, so as to achieve desired results for such parameters.
FIG. 5 depicts RFID device 500 in accordance with the prior art. RFID device 500 comprises an asymmetric dipole antenna that comprises two arms, which are depicted in FIG. 5 as 510-1 and 510-2. Each of the two arms is a metal sheet, and the two arms are of different size and shape. Also, they are not in the same plane.
RFID device 500 also comprises ground plane 515 which is also a metal sheet and is parallel to arms 510-1 and 510-2. RFID 500 also comprises load element 520, which is connected to the asymmetric dipole antenna through input-output port 540. Similar to FIG. 2, load element 520 is an electrical circuit for processing electrical signals generated by the antenna, and for generating electrical signals to be fed to the antenna. The comments that were made in conjunction with FIG. 2 regarding load element 220 also apply for load element 520.
Much like the antennas of FIGS. 1, 2, 3, and 4, efficient operation of the asymmetric dipole antenna in FIG. 5 depends on the fact that the metal sheets are good conductors at the frequency of operation of the antenna. Operational parameters of the antenna, such as impedance, radiation pattern, et cetera, are a function of the shape, size, and electrical properties of all the metal sheets, including the ground plane. It is well known in the art how to predict such operational parameters of the antenna based on the specific shape, size, and electrical properties of all the metal sheets, including the ground plane. It is also well known in the art how to adjust the shape, size and electrical properties of the metal sheets so as to achieve desired results for such parameters.
FIG. 6 depicts RFID device 600 in accordance with the prior art. RFID device 600 comprises a so-called folded dipole antenna, which is depicted in FIG. 6 as folded dipole antenna 610, and is made out of a metal sheet cut in the shape depicted in the Figure.
RFID device 600 also comprises load element 620, which is connected to the folded dipole antenna through input-output port 640. Similar to FIG. 2, load element 620 is an electrical circuit for processing electrical signals generated by the antenna, and for generating electrical signals to be fed to the antenna. The comments that were made in conjunction with FIG. 2 regarding load element 220 also apply for load element 620.
Much like the antennas of FIGS. 1-5, efficient operation of the folded dipole antenna in FIG. 6 depends on the fact that the metal sheet is a good conductor at the frequency of operation of the antenna. Operational parameters of the antenna, such as impedance, radiation pattern, et cetera, are a function of the shape, size, and electrical properties of the metal sheet. It is well known in the art how to predict such operational parameters of the antenna based on the specific shape, size, and electrical properties of the metal sheet. It is also well known in the art how to adjust the shape, size and electrical properties of the metal sheet so as to achieve desired results for such parameters.
FIG. 7 depicts RFID device 700 in accordance with the prior art. Unlike the RFID devices of FIGS. 1-6, RFID device 700 uses magnetically-coupled communications instead of radio communications. Accordingly, instead of a radio antenna, RFID device 700 comprises a magnetic antenna implemented as flat magnetic coil 710.
RFID device 700 also comprises support substrate 760 which is a thin sheet of dielectric material that provides mechanical support for the entire structure. Flat magnetic coil 710 comprises a plurality of loops of conductive material connected in series; they are depicted in FIG. 7 as conductive loops 750. The loops of flat magnetic coil 710 are typically made out of a thin strip or ribbon of metal deposited on or otherwise attached to support substrate 760. RFID device 700 has a planar structure, and FIG. 7 shows a view from outside the plane looking in a direction perpendicular to the plane.
RFID device 700 also comprises load element 720, which is connected to flat magnetic coil 710 through input-output port 740. Similar to FIG. 2, load element 720 is an electrical circuit for processing electrical signals generated by flat magnetic coil 710, and for generating electrical signals to be fed to flat magnetic coil 710. The comments that were made in conjunction with FIG. 2 regarding load element 220 also apply for load element 720, except that, in this case, the magnetic antenna implemented as flat magnetic coil 710 couples with magnetic signals instead of radio signals.
The loops of flat magnetic coil 710 are all in the same plane, and they are electrically insulated form one another because support substrate 760 is made out of a dielectric material, which is a good electrical insulator. However, input-output port 740 needs to provide a connection to both ends of the coil, and, therefore, the open end of the outermost loop must cross over all the other loops to come near the open end of the innermost loop, where input-output port 740 is located. This is accomplished by insulated crossover 770, which is a thin strip or ribbon of metal that is attached to the end of the outermost loop and crosses over all the other loops to connect to input-output port 740. Though not explicitly shown in FIG. 7, there is a layer of electrically-insulating material between insulated crossover 770 and conductive loops 750 in the area where they overlap. This layer of insulating material insures that the loops of flat magnetic coil 710 remain electrically insulated from one another.
Efficient operation of the magnetic antenna of RFID device 700, which is implemented as flat magnetic coil 710, depends on the fact that conductive loops 750 are made out of a good conductor. The operational parameters of flat magnetic coils 710, when used as a magnetic antenna, are a function of the shape, size, and electrical properties of flat magnetic coil 170. In particular, they are a function of the number of loops and of the size and shape of the area enclosed by the loops. Generally, it is desirable to have as many loops as possible that span as large an area as possible. It is well known in the art how to predict operational parameters of a magnetic antenna implemented as magnetic coil based on the specific shape, size, number of loops and electrical properties of the magnetic coil. It is also well known in the art how to adjust the shape, size, number of loops and electrical properties of the magnetic coil so as to achieve desired results for such parameters.
FIG. 8 depicts flat magnetic coil 800, which is a simplified representation of flat magnetic coil 710. This representation highlights the fact that the conductive loops, shown in the Figure as conductive loops 850, are strips or ribbons of metal that are, typically, wider than the gaps between adjacent loops. This simplified representation does not explicitly show an input-output port or an insulated crossover; however, it should be understood that such elements are typically necessary and might be present wherever a flat magnetic coil is used. These and other elements, such as a load element, necessary in practical devices that use flat magnetic coils are not explicitly shown in this simplified representation but should be understood to be present as needed. Such a simplified representation of a flat magnetic coil is used in the figures of this specification to show the presence of a flat magnetic coil, with the understanding that other elements not explicitly shown might be present, as necessary, to achieve efficient operation of the flat magnetic coil as part of a magnetic antenna.